Sunvox is out now and it is free for the N900
..with has a lot of synths,effects
and a nice gui


http://www.warmplace.ru/

you can download it from http://www.warmplace.ru/soft/sunvox/sunvox.zip...(link corrected)
and you have to install libgles1 by "apt-get install libgles1"
you may have to deactivate all repositorys except of the nokia system one..


now we have a very nice synth for the n900:-)
http://www.youtube.com/watch?v=OIFRLTqInbY